Steelix has a moderate amount of HP, and if you're decent with flipping, Slam can work wonders. Metal Slash is a bit alright (but not much), since you're dealing high damage for losing an attack unless you use Switch or something since it's retreat cost is a bit high, but works for this type of pokemon.
A card such as the D/P Steelix will definetely strengthen decks with steel as a sub-type. A 7/10 seems reasonable. The main problem here is the Metal Slash. A high retreat cost and 100 damage for losing an attack next turn just doesn't seem to work if your opponent has powerful monsters on the bench.